Silver bromide and silver bromoiodide emulsions, hereinafter collectively referred to as silver brom(oiod)ide emulsions, possess native imaging sensitivity in the ultraviolet and blue portions of the electromagnetic spectrum. Spectral sensitizing dyes have been developed to extend the imaging response of silver brom(oiod)ide throughout the visible spectrum.
With the emergence of solid state lasers as useful tools for photographic imaging applications a problem has arisen. Attempts to construct solid state lasers that emit in the visible spectrum have demonstrated that the efficiencies of solid state lasers drop sharply as emission wavelengths approach the visible. While there are many types of lasers that emit in the visible spectrum, their comparatively large and cumbersome constructions have rendered them unattractive. As a consequence, a significant need has arisen for silver halide photographic materials that are sensitive to the near infrared portion of the spectrum and, specifically, compatible with solid state laser exposure sources. As employed herein the term "near infrared" employed to indicate the 700 to 1500 nm wavelength range of the electromagnetic spectrum.
The problem that has been encountered is that the spectral sensitizing dyes that are capable of extending the photographic response of silver halide emulsions into the near infrared portion of the spectrum also desensitize the emulsions.
Dye desensitization is generally recognized and understood by those familiar with spectrally sensitized silver halide emulsions. Nevertheless, some elaboration is offered, since it is not intuitively obvious that a silver halide emulsion that shows no response to near infrared exposure in the absence of a spectral sensitizing dye, but responds in the presence of the dye, has been desensitized. Mees, The Theory of the Photographic Process, 3rd Ed., Macmillan, 1966, at page 257, explains dye desensitization and its verification. When silver halide grains are chemically sensitized, the speed of the emulsion is increased at all wavelengths. Other materials placed in or on the grains desensitize the emulsion at all wavelengths and are referred to as desensitizers. Spectral sensitizing dyes extend the sensitivity of the grains to wavelengths to which the grains lack native sensitivity, but often additionally reduce the sensitivity of the grains in the spectral region of native sensitivity. The reduction of sensitivity imparted by the dye provides an indirect indication that the dye is also reducing sensitivity in the region of spectral sensitization. The generally accepted theory stated by Mees and indicated to be consistent with results obtained by its application is that at any instant of exposure, only a minute fraction of the dye molecules on any grain are in the excited state, with the remaining, unexcited dye molecules remaining capable of adversely affecting grain sensitivity independently of the excited molecules.
Spectral sensitizing dyes are almost universally polymethine dyes, dyes that contain a chromophore extending between two terminal nuclei through a conjugated methine linkage with individual methine groups being optionally replaced with aza (--N.dbd.) linkages. While nuclei and substituent selections can influence the absorption wavelengths of the dye, polymethine dyes with absorption peaks in the blue, green, red and near infrared portions of the spectrum most notably differ in the length of their conjugated linkages between nuclei. Typically five or more methine groups join the nuclei of of polymethine dyes that exhibit an absorption peak at wavelengths greater than 700 nm. Under the topic Structure and Densensitization Mees at page 259 states, "In a given series of polymethine dyes, an increase in the length of the chain connecting the nuclei produces a rapid increase in the desensitizing effect."
It is therefore apparent that silver halide emulsions sensitized to the near infrared exhibit marked desensitization and that the increasing use of near infrared solid state lasers has increased the need for near infrared sensitized photographic emulsions exhibiting reduced dye desensitization.